My favorite post of the year! Translated: You Americans are fat slobs and force apparel manaufacturers to create large sizings so that although you are a LARGE you can actually say you wear a medium. I would guess the same is for shoe sizes as well. This put a smile on my face and is so true.
Not the way I meant it

I suppose, it's tendencies of the fit and cut on a particular market, more than anything. And style trends. A lot of my male clients are asking for fitted shirts that are just long enough to cover the belt. And preferably v-neck with short and tight sleeves
There's also an issue of non-existant sizing standards. Sizes vary greatly from brand to brand and depending on where the clothes are made. A lot of garments on AU market are made in China and in most cases run quite small - just yesterday I bought an XL top for my 14 y.o., who is generally M.
Goes both ways, I guess.
It would be so much easier for everyone if the sizes where uniform - at least within one countly, if not internationally.
In AU curently there is no standard sizing - the "recent" one was withdrawn in 2007 as it is "no longer relevant": established in 1959, the standard was based on data from a 1926 study...

As a result, designers and manufacturers are free to make up their own size specifications - often based on "vanity sizing".